The present invention relates to the stabilization of organic metal finishing additives in aqueous metal treating baths, and more particularly, to the stabilization of such organic additives by forming inclusion compounds thereof with cyclodextrins. The present invention also relates to aqueous metal treating baths in which inclusion compounds of organic metal finishing additives in cyclodextrins have been dissolved therein. The present invention provides a significant improvement in the solubility and stability of organic metal finishing additives in aqueous metal treating baths without detracting from the activity of the additive. Consequently, significantly less of the additive can be used in the treating bath to obtain the desired effect.
Organic metal finishing additives are materials that can improve the final result of many metal finishing processes. Metal finishing additives are generally used in electro or electroless plating processes to improve deposit properties such as brightness, hardness, ductility, solderability, density and other desirable characteristics. These finishing additives can be used in metal treating processes such as cleaning, pickling, etching, and the like, in order to improve the desired results obtained therewith.
In order to be effective, metal finishing additives should have good solubility in the process solvents used. They should be stable, non-volatile and long-lasting. The search for useful additives of this type has often led to organic chemical compounds which would otherwise be very useful in a metal finishing process; however, in many cases, they lack the solubility, stability or some other property, which therefore limits or negates their use.
In particular, the utility of a metal finishing additive depends upon the ability to control the additive concentration in the process solvent. This becomes problematic when the additive is unstable in the solvent, lacks solubility in the solvent, is volatile, or forms insoluble metal precipitates. At best, this problem is solved by using excess quantities of the additives to maintain an effective concentration as the additive precipitates, degrades or evaporates. At worst, the incompatibility of the process solvent may be so severe that the additive cannot be used at all.
For example, in acid copper plating solutions, copper deposits with improved brightness are obtained by adding small quantities of tetraethylthiuramidisulfide (TETD) to a conventional bright acid copper plating bath. Because TETD is insoluble in water, it must first be dissolved in methanol and then added to the plating bath as an alcoholic solution. The problem is that the additive does not remain in solution, and its beneficial effect is therefore lost as it quickly precipitates out of solution.
Flavanols are useful metal finishing additives for divalent acid tin plating baths. These compounds have a short life in the acid tin bath because they tend to form insoluble complexes with tin that results in a precipitation out of the solution. In addition, flavanols are poorly soluble in the acid tin bath, and must therefore be used at low concentrations. This poor solubility coupled with the tendency to precipitate with tin severely limits the use of flavanols in acid tin plating baths.
Acetaldehyde is another desirable additive for use in divalent acid tin plating baths. While acetaldehyde is water soluble, its bath concentration is difficult to control because of its great volatility at ambient and elevated temperatures.
Attempts to stabilize these various types of metal finishing additives in process solvents to date have at best prolonged the length of time for which an effective concentration of the additive may be maintained in the solvent. There remains a need for a means by which organic metal finishing additives may be stably incorporated into aqueous metal treating baths in a manner that provides effective concentrations with the addition of minimal quantities of the finishing additives.